I'm working on a jd 420 for my boss. A guy I work with replaced the pto clutch with a new unit, and it would'nt engage the blades at all. I tightened the 3 lock nuts on the outer plate and it worked as it should a few times (able to turn on and off) now the clutch stays engaged all the time. I've tried loosening the bolts 1/4 turns at a ti me till allmost backed off and the blades still stay engaged. Anybody have any ideas?

Well, if I understand the three locknuts part, that is NOT what should be done there. If these are ones with springs around the bolts, they DO set the clearance and brake action on the pto when turned off. What you need to do is find manual to give you clearance measurement in thousandths. Use a feeler gauge of correct size and it sticks back behind the metal and clutch surface area by each nut, one at a time to measure. Clamp the gauge in this space by tightening the nut, not overtight, just enough to hold gauge, then with slight pull on gauge start loosening the nut until gauge pulls out. Do on all three nuts. Best to re-check each when done, as moving more than one may change dimensions of others. See, there is a very light gap there when in off position. It kind of acts as a brake, yet not really tight enough to turn implement. Maybe there is a common spec size for the gauge on these, but looking in a manual for suggetions is best. The implement pulley there should be free to turn with the pto off and mower running or Not. Don't check while running. You would also have to make sure there was no binding or bending of the base plate when mounting to the engine block. That would flex and warp things and get measurements off also. Is it correct one for the model or just some generic one? Not all pto's fit the same, yet many now days are similar.

There are normally 3 slots located 120 degrees apart on the the outer frame of the PTO clutch, adjustment is with the three nuts that hold tension on the outer frame of the clutch. There are also three springs (one on each bolt) that are put on that push against the outer frame of the clutch. These springs go under the outer frame, this is how the tension is set for the clutch.